Any idea? Heat levels feel identical but the driver's side A/C isn't quite as cool.

 

Someone who knows better than me may post but for now it's probably quite like the system in my car. There are motor-driven flaps (blend doors) and also sensors so it can be any of those. May be worth cleaning a sensor or two if you can. If your car has a water valve it might be sticking (known as DCCV on the S-Type and they fail too often).

Grab the workshop manual and have a read...